Assuming the flash is in TTL mode, for a given aperture, as you raise the ISO the flash needs to put out less light to provide a "proper" exposure. For each doubling of the ISO the flash works half as hard. Thus, if you go from ISO 200 to 400, the flash needs to put out one-stop less light.

OK, still learning I guess, and good points...

My issue with this (like using auto modes on a camera) is that the flash makes decisions about the power output, the 58 is pretty good but like a camera may make wrong decisions based on overall light metering.

On a recent fashion shoot for example, I had difficulty defining the flash result because although the model was close and well lit, much of the surrounding background was not, sometimes the flash went brighter, sometimes (due to less background) it didn't, so the results varied. This meant that although the focus of the picture (the model) was normally well exposed, the background varied greatly, sometimes filled, sometimes dark making a fasion shoot set rather unbalanced. Also this meant that adjustung the camera (if i thought the model was over exposed) changed the flash output to compensate...

Setting it to manual mode meant I had a good expectation about what the flash was going to do, but if the model was closer to me (as they walked past me) then they were more or less exposed.
Just something to contend with in these situations.

At the end-of-the-day,

Flashing lots heats it up and causes the overheating problem, something you may be able to manage a little bit but not control or fix.

If I was doing this type of work a lot, and getting paid for it then a second 58 would be my solution, like a spare body, another flash in the kit would be a good backup, or useful for fill lighting...

Thanks for the technicals Mark, adds to the understanding i got from the friedman A77 book.